So called cogging is a well known feature of Permanent Magnet (PM) machines and exhibits themselves by varying machine speed and/or torque. Cogging torque is due to interaction between the permanent magnets of the rotor and the slots of the stator of PM-machine and the rotor is attracted to positions where the magnetic flux across the airgap is at a maximum. With the exception of step motors, cogging torque is undesirable because it leads to the speed variations and also to vibrations of the stator foundations, especially at low to moderate motor speeds. Most PM machines are therefore designed to minimize the cogging torque, usually through using skewed magnets or stator slots.
Despite efforts to minimize cogging torque through proper electrical and mechanical design it is quite common that PM machines have residual cogging torque amplitudes of 0.5-2% of the nominal torque. Even machines which are virtually identical, can have relative large individual differences in the cogging torque characteristics.
JP 2008000503 discloses a method for reducing torque ripples and cogging torque by adding a corrective torque to a motor. A switching means are provided for switching between addition and non-addition of the vibration control amount according to the mode of operation.
US 2008/0315810 discloses a method for reducing cogging torque where the values of corrective torque is stored in a memory. The value of the corrective torque is selected according to the operational conditions of the motor.
Modern permanent magnet machines are often controlled by variable frequency drives widely used for controlling electrical induction motors. Some of these drives also have built-in software aiming at compensating for the cogging torque. The software simply adds harmonic torque components with specified harmonic numbers, amplitudes and phases to the speed corrective torque coming from the speed controller. This compensation software has proved to work fine for a specific load and speed, but experience has shown that it fails when the speed and/or the load differ much from the tuning conditions. The reason for this is that the optimal or desired compensation signal varies much with speed and load of the motor, and the static approach with constant amplitudes and phases is not capable of adapting to new conditions.